The present invention generally relates to jitter compensation systems in rotary recording medium reproducing apparatuses, and more particularly to a system capable of effectively performing jitter compensation so that the dynamic range of a jitter compensation mechanism of a reproducing transducer which reproduces recorded signals from a rotary recording medium, can be made narrow.
Reproducing apparatuses have been reduced to practical use, in which a recorded signal in a rotary recording medium (hereinafter simply referred to as a disc) in which an information signal such as a video signal and an audio signal is recorded as a variation in geometrical configuration, is reproduced as a variation in electrostatic capacitance between an electrode of a reproducing stylus of the reproducing transducer and the recording surface of the disc. However, when irregularity (wow and flutter) exists in the rotation of the turntable which is placed with the disc to rotate the disc, eccentricity exists in the center hole of the disc, distortion is introduced in the disc upon molding of the disc, and the like, jitter (error in the time axis) is introduced in the reproduced signal. The above jitter covers a frequency range from a relatively low frequency component corresponding to the rotational period of the disc due to such causes as the above irregular rotation and eccentric center hole, to a relatively high frequency component due to such causes as the above distortion in the disc, mainly, few hundreds Hz to 1.5 kHz.
In a case where the reproduced information signal is a color video signal, the above jitter gives rise to color shading, swinging of picture image in the reproduced picture of the receiver, and the quality of the reproduced picture greatly deteriorates. This deterioration in the reproduced picture is particularly notable in a so-called direct color system in which the color signal is directly recorded without frequency conversion.
Accordingly, in a conventional system, the frequency converted chrominance sub-carrier of the reproduced color video signal is frequency-converted (heterodyne-converted) back to the original chrominance sub-carrier frequency, and the phases of this chrominance sub-carrier which is frequency-converted into the original chrominance sub-carrier frequency, and a reference signal are compared. An output error signal obtained as a result of the above phase comparison controls an oscillator which generates a signal for performing the above frequency conversion, to reduce the deviation in the time axis upon frequency conversion. Furthermore, the cantilever which is provided with the reproducing stylus at the tip end thereof, is displaced towards a relative scanning direction of the reproducing stylus with respect to the disc, by the above output error signal. Hence, in the above conventional system, jitter compensation was performed by performing the above control operation in order to prevent the introduction of deviation in the time axis.
However, in the above conventional system, the output error signal obtained as a result of the phase comparison is established so as to have a characteristic suited for controlling the oscillator which generates the signal for performing the above frequency conversion. Therefore, the above output error signal is not necessarily suited for driving a jitter compensation mechanism (so-called arm stretcher) which displaces the above cantilever. Moreover, there was a disadvantage in that even when phase compensation is performed on the above output error signal, the jitter compensation cannot be sufficiently be performed.
On the other hand, when operating the disc reproducing apparatus in synchronism with a computer, when inserting letters and the like into the reproduced picture of the receiver by the reproduced signal from the reproducing apparatus from the outside, when operating the reproducing apparatus in synchronism with a television camera, and the like, for example, the reproducing apparatus must perform the reproducing operation in synchronism with an external reference signal.
Accordingly, when one attempts to realize the above by use of the prior art, a system may be considered in which a motor for rotating the disc is rotationally controlled by the external reference signal and a detected output of a rotational phase detector for the motor. In this system, the phases of the external reference signal and a reproduced color burst signal are compared at a phase comparator, to perform jitter compensation by displacing the cantilever, by controlling the jitter compensation mechanism by the output error signal obtained from the phase comparator.
However, when the disc is rotated in synchronism with the external reference signal, the operating position of the jitter compensation mechanism differs according to the phase angle with which the disc is placed onto the turntable with respect to the above rotational phase detector for the motor. That is, according to the phase angle of the above disc, there are cases where the cantilever must perform the jitter compensation operation by displacing about a center which is greatly separated from a neutral position of the jitter compensation mechanism. A similar situation occurs when deviation exists in a transferring mechanism for the reproducing transducer, and the transferring locus of the reproducing stylus of the reproducing transducer deviated from the radial direction of the disc. Hence, in order to sufficiently perform jitter compensation even in the above described case, the jitter compensation mechanism must have a sufficiently large dynamic range. However, it is difficult to construct a jitter compensation mechanism having the above sufficiently large dynamic range, and thus, sufficient jitter compensation cannot be performed.
When attempt is made to compensate all the phase difference at the jitter compensation mechanism by forming a DC coupling between the above phase comparator and the jitter compensation mechanism, a considerably large range is required as a displacing range for the cantilever which is displaced by the jitter compensation mechanism. On the other hand, when an AC coupling is formed between the above phase comparator and the jitter compensation mechanism, normal jitter compensation cannot be performed since the range of the displacing cantilever gets out of the phase detectable range due to the stationary phase error.
In addition, in obtaining the above reproduced color burst signal, the color burst signal at a point in time after elapse of a predetermined period of time from the horizontal synchronizing signal within the NTSC system color video signal, is extracted. However, when the system of the color video signal is of the PAL system, the phase of the color burst signal at the point in time after elapse of the predetermined period from the horizontal synchronizing signal, is not constant. Accordingly, in the case where the color video signal is of the PAL system, the reproduced color burst signal obtained by the above described method cannot be used for jitter compensation.
Furthermore, when performing a special reproduction such as still, slow-motion, or quick-motion reproduction by forcibly displacing the reproducing stylus of the reproducing transducer to an adjacent track towards the outer or inner peripheral direction of the disc, there are cases where the phase shifts by one cycle of the color burst signal, when the phases of the synchronizing signal within the video signal recorded in the disc do not coincide between adjacent tracks. Hence, due to the accumulation of the shift in phase, it becomes impossible to perform a servo operation since the above accumulation exceeds the dynamic range of the jitter compensation mechanism. Therefore, the above described problem relating to the phase shift by one cycle of the color burst signal, cannot be solved by use of the above jitter compensation system.